Sigma delta modulators are well known in the art, and may be implemented in the digital domain. A conventional sigma delta modulator is simply a control loop with a degree of shaping in the frequency domain and a quantizing element in the feedback. The output is the output of the quantizer and hence is one of a small numbers of values, typically two descrete values, and the transfer function, H(s), element operates to shape the noise that the quantizer introduces into the loop. The quantizer is caused to operate at discrete times, where it is clocked by some input signal and creates a discrete set of outputs at defined times.
In one conventional sigma delta design, the output may be used to create an analog signal via a simple switch between two levels, or via a semi-analog finite impulse response filter. Such a design suffers from a problem of mismatched edges in the analog digital to analog converter (“DAC”) output. A common manifestation of this problem is the creation of harmonic distortion in the analog signal. Some solutions address fixing the “frequency of edges” to make the frequency invariant with respect to the signal content in the sigma delta stream. Achieving this fixed frequency of edges removes the distortion due to mismatched edge rates in the analog circuit. However, any conventional approach to addressing the frequency of edges does so at the expense of the achievable full scale range of the DAC.
Therefore, there exists a need for an improved sigma delta modulator with reduced distortion. As will be seen, the invention accomplishes this in an elegant manner.